Obviously a single query is faster than a join, but it's very hard to guarantee that you'll always be making that single query.
Data inconsistency is not about concurrency. See this example on Wikipedia illustrating why 3NF is necessary: https://en.wikipedia.org/wiki/Third_normal_form#.22Nothing_b...
In that example, "Al Fredrickson" can potentially have 2 birthdays even in an entirely single threaded app.
I'm really curious to hear a use case where Mongo was actually significantly faster than Postgres. Could you give a toy example that illustrates the flavor of the problem?